The transfer function editor in the Probe panel includes a Histogram display. This histogram reflects the values of the selected variable in the full 2D or 3D box of the probe, and takes into account the probe orientation as well.
If the probe is not planar (unchecking the "Planar Probe" checkbox), the Probe can be expanded to a full 3D box, which can then be used to histogram the values of 3D variables in an arbitrarily oriented box.
More information about histogramming in VAPOR is provided under Data Histograms.
